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Abstract

Electostatic pen digitizer tablets usually have a high noise to signal ratio which make them prone to false position sensing. Described is a method of synchronizing to and time offsetting the LCD noise from the pen digitizer A/D input signal using a sample and hold circuit as a time windowing device.

Country

United States

Language

English (United States)

This text was extracted from an ASCII text file.

This is the abbreviated version, containing approximately
52% of the total text.

Electostatic
pen digitizer tablets usually have a high noise to
signal ratio which make them prone to false position sensing.
Described is a method of synchronizing to and time offsetting the LCD
noise from the pen digitizer A/D input signal using a sample and hold
circuit as a time windowing device.

Tablet
computers with digitizing screens using pen radiated
input are susceptible to noise coupling from the LCD driving signals,
power supply switching signals, backlight driver, and other system
digital noise sources. The noise
coupling mechanism is mostly
electrostatic and because of the large digitizing surface area much
noise is picked up and transferred to the low level input circuits of
the digitizer sensor amplifiers. The pen
signal being
electrostaticly coupled into the digitizer tablet also has to
overcome the large noise signal levels.
The pen location signal
starts out at a disadvantage because it is such a small level signal
when compared to the noise signal. A
method may be employed by which
the time and amplitude characteristics of the noise signals can be
used to advantage in overcoming the high noise to signal ratios.

By gating the
input signal off (using a sample and hold
circuit) during the periodic noise burst the high level noise induced
by the Liquid Crystal Display (LCD) device would be reduced. This
would prevent the input amplifiers from saturating while allowing
higher amplifier gains and better Signal-to-Noise (S/N) ratios.
Refer to Fig. 1 for the following discussion.
The LCD contrast
control signal runs at approximately 1.2 KHz and is capacitively
coupled into the digitizer tablet at each fast edge transition. This
produces a noise frequency of 2.4 KHz at an amplitude of
approximately 25 Vpp which is variable in level with the contrast
control adjustment. It can be in the
range of 12 to 25 volts with
the higher voltage coupling more noise.
The pen signal is typically
in the tens of milli...